For Youth football – The duration of play shall be as follows unless it is mutually agreed by all parties to reduce the time. For under 11 and under 12, 30 minutes each half; for under 13, 14, 35 minutes each half and under 15 and under 16, 40 minutes each half; under 17 and under 18, 45 minutes each half.
The minimum time for any game will not be less than 20 minutes each half for players in the under 14 age group and below and 25 minutes each half for all other age groups.
No player participating in an under 17 division or lower age group shall be permitted to play more than one game or, in the event the competition allows the playing of a double-header, ie: two separate matches, 100 minutes per day in this Competition.
thats an extract from Kent FA standard code of rules for youth competitions